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The majority of the series takes place in the mid to late 1930 s. Using on screen references "present day" takes place between 1934 (Four and Twenty Blackbirds) and 1939 (the big four). With two episodes, The Mysterious affair at styles, and Curtain book ending the series by being set in 1917 and 1949. The Labours of Hercules take place in 1939 / 1940 and is set twenty years after the episode the double clue, retconning "The Double Clue" to be set in the 1920s.
It is impossible to draw up an accurate timeline of episodes as many, like the above contradict. Edit (Coming Soon)
There are 5 episodes which bear little relation to the source material they are based on, at least two of these have a totally new plot and characters, these are: The case of the missing will Appointment with Death (The murderer has the same name as in the book, but another profession and motivation. And a supporting criminal, too) Cards on the Table (One of the suspects are guilty of homicide in the book, but innocent in the adaption. This character drowns by accident in the book, but ends up with a romance in the adaption) The Big Four (only loosely based on the book) The labours of Hercules (loosely based on a short story and expanded)
The other adaptations are generally faithful to Christies works, the short stories sometimes have Japp, Hastings and / or Miss Lemon added with an extra red herring subplot for them to investigate, while the longer novels usually have one or two character subplots conjoined or removed. On Occasion a minor recurring character, for example Superintendent Spence, will be replaced with a new character due to actor availability. Because the books were not adapted in published order there are some character plots and themes which are removed from the earlier adaptations and present in the later ones, these included Hastings marriage, his move to the Argentine and Poirots inner battle with mortality, religion, law and morals.
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All 34 Novels and 36 of the 37 canon short stories have been filmed. The one missing is The Lemesurier Inheritance.
There are several other short stories which Christie later rewrote into novels, or rewrote to have one of her other detectives to solve, For example, The Mystery of the Baghdad Chest became The Mystery of the Spanish Chest. The Regatta Mystery was later expanded into a more famous telling sharing the same title by Christie and featuring Parker Pyne as the detective.
Additionally, another story not adapted into an episode is the play Black Coffee. In 2012, David Suchet performed a rehearsed reading of Black Coffee, produced and presented by The Agatha Christie Theatre Company, in aid of Chichester Festival Theatre's restoration fund. Edit (Coming Soon)